.\"  Copyright (C) 2003  Davide Libenzi
.\"  Davide Libenzi <davidel@xmailserver.org>
.\"
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.\" %%%LICENSE_END
.\"
.TH EPOLL_CTL 2 2016-10-08 "Linux" "Linux Programmer's Manual"
.SH NAME
epoll_ctl \- control interface for an epoll file descriptor
.SH SYNOPSIS
.B #include <sys/epoll.h>
.sp
.BI "int epoll_ctl(int " epfd ", int " op ", int " fd \
", struct epoll_event *" event );
.SH DESCRIPTION
This system call performs control operations on the
.BR epoll (7)
instance
referred to by the file descriptor
.IR epfd .
It requests that the operation
.I op
be performed for the target file descriptor,
.IR fd .

Valid values for the
.I op
argument are:
.TP
.B EPOLL_CTL_ADD
Register the target file descriptor
.I fd
on the
.B epoll
instance referred to by the file descriptor
.I epfd
and associate the event
.I event
with the internal file linked to
.IR fd .
.TP
.B EPOLL_CTL_MOD
Change the event
.I event
associated with the target file descriptor
.IR fd .
.TP
.B EPOLL_CTL_DEL
Remove (deregister) the target file descriptor
.I fd
from the
.B epoll
instance referred to by
.IR epfd .
The
.I event
is ignored and can be NULL (but see BUGS below).
.PP
The
.I event
argument describes the object linked to the file descriptor
.IR fd .
The
.I struct epoll_event
is defined as:
.sp
.in +4n
.nf
typedef union epoll_data {
    void        *ptr;
    int          fd;
    uint32_t     u32;
    uint64_t     u64;
} epoll_data_t;

struct epoll_event {
    uint32_t     events;      /* Epoll events */
    epoll_data_t data;        /* User data variable */
};
.fi
.in

The
.I events
member is a bit mask composed using the following available event
types:
.TP
.B EPOLLIN
The associated file is available for
.BR read (2)
operations.
.TP
.B EPOLLOUT
The associated file is available for
.BR write (2)
operations.
.TP
.BR EPOLLRDHUP " (since Linux 2.6.17)"
Stream socket peer closed connection,
or shut down writing half of connection.
(This flag is especially useful for writing simple code to detect
peer shutdown when using Edge Triggered monitoring.)
.TP
.B EPOLLPRI
There is urgent data available for
.BR read (2)
operations.
.TP
.B EPOLLERR
Error condition happened on the associated file descriptor.
.BR epoll_wait (2)
will always wait for this event; it is not necessary to set it in
.IR events .
.TP
.B EPOLLHUP
Hang up happened on the associated file descriptor.
.BR epoll_wait (2)
will always wait for this event; it is not necessary to set it in
.IR events .
Note that when reading from a channel such as a pipe or a stream socket,
this event merely indicates that the peer closed its end of the channel.
Subsequent reads from the channel will return 0 (end of file)
only after all outstanding data in the channel has been consumed.
.TP
.B EPOLLET
Sets the Edge Triggered behavior for the associated file descriptor.
The default behavior for
.B epoll
is Level Triggered.
See
.BR epoll (7)
for more detailed information about Edge and Level Triggered event
distribution architectures.
.TP
.BR EPOLLONESHOT " (since Linux 2.6.2)"
Sets the one-shot behavior for the associated file descriptor.
This means that after an event is pulled out with
.BR epoll_wait (2)
the associated file descriptor is internally disabled and no other events
will be reported by the
.B epoll
interface.
The user must call
.BR epoll_ctl ()
with
.B EPOLL_CTL_MOD
to rearm the file descriptor with a new event mask.
.TP
.BR EPOLLWAKEUP " (since Linux 3.5)"
.\" commit 4d7e30d98939a0340022ccd49325a3d70f7e0238
If
.B EPOLLONESHOT
and
.B EPOLLET
are clear and the process has the
.B CAP_BLOCK_SUSPEND
capability,
ensure that the system does not enter "suspend" or
"hibernate" while this event is pending or being processed.
The event is considered as being "processed" from the time
when it is returned by a call to
.BR epoll_wait (2)
until the next call to
.BR epoll_wait (2)
on the same
.BR epoll (7)
file descriptor,
the closure of that file descriptor,
the removal of the event file descriptor with
.BR EPOLL_CTL_DEL ,
or the clearing of
.B EPOLLWAKEUP
for the event file descriptor with
.BR EPOLL_CTL_MOD .
See also BUGS.
.TP
.BR EPOLLEXCLUSIVE " (since Linux 4.5)"
Sets an exclusive wakeup mode for the epoll file descriptor that is being
attached to the target file descriptor,
.IR fd .
When a wakeup event occurs and multiple epoll file descriptors
are attached to the same target file using
.BR EPOLLEXCLUSIVE ,
one or more of the epoll file descriptors will receive an event with
.BR epoll_wait (2).
The default in this scenario (when
.BR EPOLLEXCLUSIVE
is not set) is for all epoll file descriptors to receive an event.
.BR EPOLLEXCLUSIVE
is thus useful for avoiding thundering herd problems in certain scenarios.

If the same file descriptor is in multiple epoll instances,
some with the
.BR EPOLLEXCLUSIVE
flag, and others without, then events will be provided to all epoll
instances that did not specify
.BR EPOLLEXCLUSIVE ,
and at least one of the epoll instances that did specify
.BR EPOLLEXCLUSIVE .

The following values may be specified in conjunction with
.BR EPOLLEXCLUSIVE :
.BR EPOLLIN ,
.BR EPOLLOUT ,
.BR EPOLLWAKEUP,
and
.BR EPOLLET .
.BR EPOLLHUP
and
.BR EPOLLERR
can also be specified, but this is not required:
as usual, these events are always reported if they occur,
regardless of whether they are specified in
.IR events .
Attempts to specify other values in
.I events
yield an error.
.B EPOLLEXCLUSIVE
may be used only in an
.B EPOLL_CTL_ADD
operation; attempts to employ it with
.B EPOLL_CTL_MOD
yield an error.
If
.B EPOLLEXCLUSIVE
has been set using
.BR epoll_ctl (),
then a subsequent
.B EPOLL_CTL_MOD
on the same
.IR epfd ",\ " fd
pair yields an error.
A call to
.BR epoll_ctl ()
that specifies
.B EPOLLEXCLUSIVE
in
.I events
and specifies the target file descriptor
.I fd
as an epoll instance will likewise fail.
The error in all of these cases is
.BR EINVAL .
.SH RETURN VALUE
When successful,
.BR epoll_ctl ()
returns zero.
When an error occurs,
.BR epoll_ctl ()
returns \-1 and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EBADF
.I epfd
or
.I fd
is not a valid file descriptor.
.TP
.B EEXIST
.I op
was
.BR EPOLL_CTL_ADD ,
and the supplied file descriptor
.I fd
is already registered with this epoll instance.
.TP
.B EINVAL
.I epfd
is not an
.B epoll
file descriptor,
or
.I fd
is the same as
.IR epfd ,
or the requested operation
.I op
is not supported by this interface.
.TP
.B EINVAL
An invalid event type was specified along with
.B EPOLLEXCLUSIVE
in
.IR events .
.TP
.B EINVAL
.I op
was
.B EPOLL_CTL_MOD
and
.IR events
included
.BR EPOLLEXCLUSIVE .
.TP
.B EINVAL
.I op
was
.B EPOLL_CTL_MOD
and the
.BR EPOLLEXCLUSIVE
flag has previously been applied to this
.IR epfd ",\ " fd
pair.
.TP
.B EINVAL
.BR EPOLLEXCLUSIVE
was specified in
.IR event
and
.I fd
refers to an epoll instance.
.TP
.B ELOOP
.I fd
refers to an epoll instance and this
.B EPOLL_CTL_ADD
operation would result in a circular loop of epoll instances
monitoring one another.
.TP
.B ENOENT
.I op
was
.B EPOLL_CTL_MOD
or
.BR EPOLL_CTL_DEL ,
and
.I fd
is not registered with this epoll instance.
.TP
.B ENOMEM
There was insufficient memory to handle the requested
.I op
control operation.
.TP
.B ENOSPC
The limit imposed by
.I /proc/sys/fs/epoll/max_user_watches
was encountered while trying to register
.RB ( EPOLL_CTL_ADD )
a new file descriptor on an epoll instance.
See
.BR epoll (7)
for further details.
.TP
.B EPERM
The target file
.I fd
does not support
.BR epoll .
This error can occur if
.I fd
refers to, for example, a regular file or a directory.
.SH VERSIONS
.BR epoll_ctl ()
was added to the kernel in version 2.6.
.\" To be precise: kernel 2.5.44.
.\" The interface should be finalized by Linux kernel 2.5.66.
.SH CONFORMING TO
.BR epoll_ctl ()
is Linux-specific.
Library support is provided in glibc starting with version 2.3.2.
.SH NOTES
The
.B epoll
interface supports all file descriptors that support
.BR poll (2).
.SH BUGS
In kernel versions before 2.6.9, the
.B EPOLL_CTL_DEL
operation required a non-null pointer in
.IR event ,
even though this argument is ignored.
Since Linux 2.6.9,
.I event
can be specified as NULL
when using
.BR EPOLL_CTL_DEL .
Applications that need to be portable to kernels before 2.6.9
should specify a non-null pointer in
.IR event .

If
.B EPOLLWAKEUP
is specified in
.IR flags ,
but the caller does not have the
.BR CAP_BLOCK_SUSPEND
capability, then the
.B EPOLLWAKEUP
flag is
.IR "silently ignored" .
This unfortunate behavior is necessary because no validity
checks were performed on the
.IR flags
argument in the original implementation, and the addition of the
.B EPOLLWAKEUP
with a check that caused the call to fail if the caller did not have the
.B CAP_BLOCK_SUSPEND
capability caused a breakage in at least one existing user-space
application that happened to randomly (and uselessly) specify this bit.
.\" commit a8159414d7e3af7233e7a5a82d1c5d85379bd75c (behavior change)
.\" https://lwn.net/Articles/520198/
A robust application should therefore double check that it has the
.B CAP_BLOCK_SUSPEND
capability if attempting to use the
.B EPOLLWAKEUP
flag.
.SH SEE ALSO
.BR epoll_create (2),
.BR epoll_wait (2),
.BR poll (2),
.BR epoll (7)
